1. Field of the Invention
The present invention relates to a digital signal processing device to execute a time-sharing multiplex or separation of two kinds of digital signals having different data rates.
2. Related Background Art
To transmit a plurality of kinds of digital signals, ordinarily, a time-sharing multiplex system is used and, in the case where data rates of the digital signals are different, time bases of the digital signals are compressed by time base compressing circuits, thereby matching the transmission data rates. FIG. 1 is a block diagram showing a conventional general construction of a device to time-sharing multiplex a video signal and an audio signal. A video signal is supplied to an input terminal 12. An audio signal is supplied to an input terminal 10. The video signal supplied to the input terminal 12 is converted into the digital signal by a video signal processing circuit 16. The audio signal supplied to the input terminal 10 is converted into the digital signal by an audio signal processing circuit 14. In the video signal and the audio signal, the data rate of the digital signal derived from the video signal of a wide signal band is higher than that of the digital signal derived from the audio signal. Therefore, output signals of the signal processing circuits 14 and 16 cannot be directly multiplexed. Time bases of those output signals are respectively converted by time base conversion circuits 18 and 20 and, thereafter, time base converted output signals of the time base conversion circuits 18 and 20 are multiplexed by a multiplexer 22. To make a data rate of the multiplexed digital signal coincide with a transmission rate of a transmission line, a time base of the multiplexed digital signal is further converted by a time base conversion circuit 24 and an output signal of the time base conversion circuit 24 is sent from an output circuit 26 to the transmission line. Each of the time base conversion circuits 18, 20, and 24, is constructed by a random access memory (RAM). The writing and reading operations of each RAM are controlled by a memory control circuit 28.
As mentioned above, in the case of multiplexing the data of different data rates and transmitting, a number of time base conversion circuits, namely, a number of memories are necessary and a scale of hardware increases, so that it becomes a cause to obstruct the realization of a miniaturization of the apparatus and low costs.